Calculator
Interest Calculator
Enter your starting balance, any regular contributions, and a rate of return to see your ending balance, how much of it is interest, and what that's worth after taxes and inflation.
Results
Fill in your numbers below, then press Calculate to see your results.
Investment
Initial investment is the lump sum you're starting with. Add an annual and/or monthly contribution on top of that for however long you plan to keep contributing.
Beginning means each contribution starts earning right away; end means it lands after that period's interest has already been credited. Beginning always produces a slightly larger balance.
Contribute at the beginning or end of each period
Growth
Compound is how often interest is calculated and added to the balance. More frequent compounding grows a bit faster at the same nominal rate.
Tax rate is withheld from interest as it's earned, which also shrinks what compounds going forward. Inflation rate doesn't touch the balance — it only re-expresses the final figure in today's buying power.
Investment length
For educational purposes only. Results are estimates and do not constitute financial, tax, or legal advice. Consult a qualified professional before making any financial decisions.
How to use it
Start with your initial investment, then add whatever you plan to contribute — a fixed amount each year, each month, or both.
Press Calculate for your projection, or flip on Real-Time Calculation to have it update as you type. Hover the ? beside any section for what it does.
Try raising the monthly contribution by $50 and watch how much of the ending balance shifts from principal to interest — that shift is compounding at work.
Assumptions
- The nominal interest rate is converted to an equivalent monthly rate based on your chosen compounding frequency, then applied every month — so the yearly figures land exactly where compounding at that frequency would put them.
- Tax, if set, is withheld from interest as it's earned each month, which also reduces what compounds afterward. This models a taxable account, not tax-deferred growth.
- The inflation rate never touches the balance itself — it only re-expresses the final ending balance in today's purchasing power.
- Assumes contributions and the rate of return stay constant for the whole term, with no withdrawals or fees.